Technology for writing characters and symbols on a sheet medium such as paper using laser are practically applied in various fields. For example, such technology may be used to facilitate drawing characters and other objects on labels of containers used at factories and other sites. Also, practical applications are developing for rewritable thermal paper (referred to as “rewritable paper” hereinafter) that can have objects drawn and erased multiple times. For example, in the context of applying this technology to containers used in product distribution, since the destination of a container is net necessarily the same each time it is dispatched, the above technology may be used to erase characters drawn on a label so that new characters can be drawn on the same label. In this way, the need to replace the label may be reduced.
It is noted that the color of rewritable paper may be erased at a certain temperature, and the thermal paper may develop color when en even higher temperature is applied thereon. However, when excessive heat is applied, the rewritable paper may be prone to degradation. That is, the properties of the rewritable paper may be altered, the lifecycle of the rewritable paper may be reduced, and/or the rewritable paper may lose its ability to completely erase its color, for example. Excessive heating may occur when heat is further applied to a region that is already at a relatively high temperature. In the case of drawing objects on a label, a region where characters and symbols cross and/or a region in which adjacent parallel lines are drawn to fill in the region may be prone to degradation due to excessive heating.
In consideration of the above, control techniques are known for controlling a laser irradiating apparatus to refrain from applying excessive heat on rewritable paper (e.g., see Japanese Laid-Open Patent Publication Nos. 2008-62506 and 2011-116116).
Japanese Laid-Open Patent Publication No. 2008-62506 discloses a control method for controlling the time between the start of drawing a first line and the end of drawing a second line or the overlapping width of the first line and the second line upon drawing parallel lines that are adjacent to each other.
Japanese Laid-Open Patent Publication No. 2011-116116 discloses dividing an image plotting target into plural rows (line segments) and controlling the laser output and/or the drawing speed for each row. In this way, excessive heating may be prevented, coloration characteristics of the rewritable paper may be improved, and the image quality of a colored-in region may be improved.
However, merely dividing an image plotting target into plural rows as described above may not adequately improve the image quality of the colored-in region.
FIG. 1 is a diagram illustrating a problem that may be encountered when a stroke to be drawn is relatively short. The two arrows pointing upwards and downwards shown at the left side of FIG. 1 represent strokes (lines) of an object to be drawn. A laser irradiating apparatus draws the left stroke (arrow pointing downward) first and then the right stroke (arrow pointing upward). Thus, when drawing the right stroke, residual heat may remain from drawing the left stroke.
The arrows at the right side of FIG. 1 illustrate an exemplary manner of controlling the scanning speed for drawing the right stroke. In the related art, one stroke is divided into a given number of line segments and the scanning speed is adjusted for each line segment. In the illustrated example of FIG. 1, the right stroke is divided into four line segments. It is noted that the scanning speed is not controlled segment-by-segment for the left stroke. The left stroke is broken into three segments in FIG. 1 to illustrate the differences in residual heat affecting the line segments; however, the scanning speed is maintained constant when drawing the left stroke.
As is shown in FIG. 1, the amount of residual heat around the starting point of the left stroke is relatively small, whereas the amount of residual heat near the end point of the stroke is relatively large. Thus, when drawing the right stroke that is divided into four line segments, the lower line segment is arranged to be drawn at a faster scanning speed than the higher line segment. Assuming the scanning speeds for drawing the four line segments of the right stroke are denoted as drawing speeds S(1)-S(4) as is shown in FIG. 1, where S(4) represents the normal drawing speed, their relationship may be represented as follows:S(1)>S(2)>S(3)>S(4).
In FIG. 1, the right stroke is divided into four line segments even though the stroke is relatively short. Thus, the line segment including the end point of the right stroke is drawn at the normal drawing speed S(4) even though heat may still remain around the starting point of the left stroke. In this case, the temperature of the rewritable paper at the upper portion of the right stroke may rise to an undesirably high level.
FIG. 2 is a diagram illustrating a problem that may be encountered when a stroke to be drawn is relatively long. In FIG. 2, even though the strokes are longer than those of FIG. 1, the right stroke is still divided into four line segments. It is noted that the scanning speed for drawing the left stroke is not controlled segment-by-segment as in the example of FIG. 1.
In FIG. 2, since the strokes are relatively long, residual heat does not remain around the starting point of the left stroke, the amount of residual heat at the middle portion of the left stroke is relatively small, and the amount of residual heat around the end point of the left stroke is relatively large. However, the line segments of the right stroke are drawn at the drawing speeds S(1)-S(4) in a manner similar to FIG. 1, where the lower line segment is drawn at a higher speed than the higher line segment. In this case, the end portion of the line segment drawn at the fastest drawing speed S(1) is located near the middle portion of the left stroke where the amount of residual heat is smaller, and as a result, the temperature of the rewritable paper at this portion may not rise to a sufficient level. It is noted that the same problem occurs at the end portion of the line segment drawn at drawing speed S(2) that is located near the middle portion of the left stroke where the amount of residual heat is small and the end portion of the line segment drawn at speed S(3) that is located near the starting point of the left stroke where there is no residual heat.
As can be appreciated, the drawing time for drawing an adjacent stroke and the impact of residual heat from drawing the adjacent stroke differ depending on the length of the stroke. However, the related art does not contemplate a method for determining the appropriate laser output level and drawing speed based on such factors.